Circuit for avoiding too low a potential difference between control grid and cathode of a cathode-ray tube



March 14, 1961 T. POORTER 2,975,327

CIRCUIT FOR AVOIDING TOO LOW A POTENTIAL DIFFERENCE BETWEEN CONTROL GRID AND CATHODE. OF A CATHODE-RAY TUBE Filed Dec. 1, 1959 INVENTOR TEUNIS POORTER CIRCUIT FOR AVOIDING T LOW A PGTENTIAL DIFFERENCE BETWEEN CGNTRUL GRID AND CATHGDE OF A CATHODE-RAY TUBE Tennis Poorter, Eindhoven, Netherlands, assignor to North American Philips Company, Inc, New York, N.Y., a corporation oft Delaware Filed Dec. 1, 1959, Ser. No. 856,555

Claims priority, application Netherlands Dec. 2, 1958 3 Claims. (Cl. 315-30) This invention relates to circuits for avoiding too low a potential difference between control grid and cathode of a cathode-ray tube, intended more particularly for reproducing television signals, in which the circuit for supplying the signals to be reproduced to a control electrode of the cathode-ray tube includes a unilaterally conductive element having a polarity such and a bias such that those signal portions which exceed a given signal level cannot bring about, or at least to a much lesser extent, a decrease in the potential difference between control grid and cathode of the cathode-ray tube, an adjustable direct voltage being applied to the said control electrode or a further control electrode of the cathode-ray tube for the purpose of adjusting the background brightness of the cathode-ray tube.

For several reasons it is desirable to avoid too low a value of the said potential difference in a cathode-ray tube.

Firstly, too low such a value may bring about an unduly great electron current and, consequently, cause damage to the fluorescent screen of the cathode-ray tube.-

Secondly, too low such a value may give rise to the occurrence of grid current at the control grid of the cathode-ray tube, so that rectification occurs at this control grid and hence the direct voltage determining the background brightness undergoes a variation in the sense that a direct-voltage component is apparently added to the signal to be reproduced, which direct-current component has a polarity decreasing the stream of electrons. Consequently, improper reproduction of the signal occurs and the stream of electrons may, at certain moments, even be suppressed completely. What is most objectionable of this phenomenon is that the time constant of the control-grid circuit is then such that this variation in potential is maintained for a certain period and that reproduction is influenced all this time.

In colour television reproducing device of the type used nowadays, the streams of electrons co-acting with the blue and red phosphors are usually required to be controlled more strongly than the stream of electrons coacting with the green phosphor. It is thus possible that a very bright spot in the scene to be reproduced as a result of the phenomena above described, causes a decrease of the streams of electrons co-acting with the blue and red phosphors so that the image reproduced contains too much green either locally or as a whole, dependently upon the value of the above-mentioned time constant.

The occurrence of grid current may also cause serious damage to the cathode so that the length of life of the cathode-ray tube is drastically restricted.

Damage to the fluorescent screen and the occurrence of grid current could naturally be avoided by means of a sufliciently low value of the signal to be supplied to the cathode-ray tube. However, since very bright spots in the scene to be reproduced only occur comparatively seldom and hence contribute very little to the quality of the image, it is not particularly attractive for such bright spots to be reproduced correctly at the expense of the mean brightness of the image.

An object of the circuits described in the preamble is to avoid damage to the fluorescent screen and the occurrence of grid current without having resource to a reduction of the signal such that even the values of the signal which occur with very bright spots do not cause the potential dilierence between control grid and cathode of the cathode-ray tube to exceed a given minimum. The circuits are based upon the above-mentioned recognition that the high values of the signal corresponding to very bright spots in the image are little contributive only to the quality of the image.

Circuits of known type have the disadvantage that the control of the background brightness may disturb the proper performance. If, starting from a value of the background brightness at which the limiting level adjusted by the bias of the unilaterally conductive element has a correct value, this background brightness is increased, the said potential difference still can assume an undesirably low value despite the limitation of the signal to be reproduced. If the background brightness is decreased, the signal to be reproduced is limited to an unnecessarily low value.

An object of the invention is to obviate these disadvantages.

For this purpose, the circuit according to the invention is characterized in that means are provided which, upon controlling the direct voltage for the adjustment of the background brightness, also cause a variation in the bias of the unilaterally conductive element to an extent such that upon an increase or decrease of the background brightness, the said signal level becomes lower and higher, respectively, with respect to the signal to be reproduced.

In order that the invention may be readily carried into efiect, one embodiment will now be described in detail, by way of example, with reference to the accompanying drawing in which:

Fig. 1 shows a current-voltage characteristic of a cathode-ray tube, and

Fig. 2 shows one embodiment of a circuit according to the invention.

In Fig. l, the current i which is active -at the fluorescent screen ofa cathode-ray tube is plotted as a function of the potential difference V between control grid and cathode of the tube. If the background brightness of the tube is adjusted to a value determined by a potential difference V between control grid and cathode, the signal to be reproduced controls the tube in accordance with curve 1, if the signal had not been limited beforehand. The peak 2 in the signal which corresponds to a very bright point in the image to be reproduced then causes a decrease of V to an extent such that grid current i starts to flow. This value of V is indicated by V in Fig. 1. Such a decrease of V,; to a level lower IE3. than V is highly undesirable for the reasons previously explained. The signal 1, before being supplied to the tube, is therefore limited to a signal level such that such undesirable decrease of V is avoided.

Such a limitation is obtained, for example, by connecting parallel to the signal input of the tube a unilaterally conductive element provided with a suitable bias active in the blocking direction, which element becomes conducting only if the signal assumes a value higher than the said bias, which bias in the case under consideration thus must be less than, or may be at the most equal to, V V

If the background brightness is decreased .by varying the direct voltage determining the potential difierence V to an extent such that this potential difference increases, for example assumes the value V which value corresponds to the Cut-Off point of the tube, the tube is controlled to an unnecessarily low extent by the signal which is limited to a signal level corresponding to a potential difference at the most equal to V V However, if the background brightness is increased and hence V is displaced more to the right in the figure, it will be evident that the above-mentioned limitation of the signal doe not avoid the occurrence of grid current.

According to the invention, upon increasing or decreasing the background brightness, the saidlimitation level is chosen to be lower and higher, respectively, with respect to the signal to be reproduced.

This is achieved, for example, by providing for the potentiometer determining the level of the background brightness and a potentiometer which determines the bias of the unilaterally conductive element to be mechanically coupled together in a suitable manner, for example, by arranging the two potentiometers on the same rotary shaft in a manner such that upon increase of the potential difference V the potential difference less than or at most equal to V -V is increased by the same amount, and that upon decrease of the potential difierence V the potential difference determining the limitation level is decreased by the same amount.

Fig. 2 shows one embodiment of a circuit according to the invention in which upon variation of one potentiometer at the same time the background brightness and the limitation level are varied In this figure, 3 indicates a cathode-ray tube, a cathode 4 of which has supplied to it the signal to be reproduced which occurs at an anode resistor 10 of a discharge tube 5 in which the signal undergoes a last amplification. The cathode 4 of the tube is also coupled to the cathode of a unilaterally conductive element 6, the anode of which is connected to a cathode 11 of a discharge tube 7 which is circuited as a cathode follower and hence has a low output impedance for direct voltage. The cathode of tube 7 is also connected to earth via a resistor 8 and. a decoupling capacitor 9 connected parallel thereto. A direct voltage is applied to a control grid 12 of tube 7. For this purpose, said control grid is coupled via a resistor 13 to a potentiometer 14, one end of which is connected to earth and the other end of which is connected to the positive terminal of a battery (not shown). The control grid 12 is decoupled for alternating currents via a capacitor 15. The control grid 12 is also connected to a control grid 16 of cathoderay tube 3.

A given potential at control grid 12 of tube 7 and at control grid 16 of cathode-ray tube 3 corresponds to a determined position of potentiometer 1 4. Said potential at control grid 16 determines the background brightness of the cathode-ray tube and at control grid 12 the cathode current of tube '7 and hence the positive direct voltage at the cathode 11 of this tube. This positive direct voltage is always higher than the potential at control grid 12. V

i The last-mentioned direct voltage isdeterminative of that value of the signal set up across the anode resistor 1d of tube 5 which releases the unilaterally conductive element, in other words, said direct voltage is determinative of the signal level above which the signal to be reproduced cannot bring about, or at least to a much lesser extent, a decrease in the potential difference between control grid and cathode of the cathode-ray tube. The tube '7 and the resistor 8 are preferably chosen to be such that with a given background brightness, and hence with a given V the signal thus limited just does not reach the value V V If, in order to increase the background brightness, the potential at control grid 16 is made less negative, the voltage at control grid 12 at the same time becomes less negative, resulting in a greater cathode current in tube 7. This results in a substantially equal variation in the potential at cathode 11 and hence a lower bias across the element 6. The signal set up across anode resistor 10 of tube 5 is thus limited to a lower active value so that, even upon increase of the background brightness, the potential difierence between control grid and cathode of tube 3 does not become lower than the value V g If, however, in order to decrease the background brightness, the potential at control grid 16 is made more negative, thepositive potential at cathode 11 of tube 7 also decreases by approximately the same value. Consequently, higher values of the signal to be reproduced may be active, in the cathode-ray tube so that this signal is not limited to an unnecessarily low value.

It is to be noted that in a colour television reproducing device, in view of the fact that the electron stream co-acting with the green phosphor need be controlled to a comparatively much smaller extent that the electron streams co-acting with the red and blue phosphors, it is usually necessary only to provide those sections with a circuit according to the invention which provide for the last two electron streams.

It is also to be noted that in the foregoing only the case has been considered that the occurrence of grid currents must be avoided. Before grid current occurs, the stream of electrons may already be such that the fluorescent screen is liable to be damaged. The limitation level for the signals to be reproduced is then preferably chosen to be lower than that which corresponds to a potential difference V -V What is claimed is:

1. A circuit for controlling the potential diflference between the control grid and cathode of a cathode ray tube comprising a source of signals, means applying said signals to said cathode, a unilaterally conductive element having a cathode connected to said cathode of said cathode ray tube and an anode connected to ground reference potential by way of resistance means, a source of variable positive direct voltage, a discharge tube having control grid, anode, and cathode electrodes, said discharge tube being connected as a cathode follower, with the control grid electrode being connected to said source of variable direct voltage and to the control grid of said cathode ray tube, and the cathode of said discharge tube being connected to said anode of said unilaterally conductive element.

2. A circuit for controlilng the potential difierence between first and second control electrodes of a cathode ray .tube, said circuit comprising a source of signals, means applying said signals to said first control electrode, a source of variable direct voltage, means applying said variable direct voltage to said second control electrode to control the background brightness of said cathode ray tube, an amplifying device having a control terminal and an output terminal, means applying said variable direct voltage to said control terminal, a unilaterally conductive element, and means connecting said unilaterally conductive element between said first control electrode and said output terminal with a polarity to apnea? limit the maximum electron beam current in said cathode ray tube.

3. A circuit for controlling the potential difference between first and second control electrodes of a cathode ray tube, said circuit comprising a source of signals, means applying said signals to said first control electrode, a source of variable direct voltage, means applying said variable direct voltage to said second control electrode to control the background brightness of said cathode ray tube, an electron discharge tube having a control grid, a cathode, and an anode, means applying said variable direct voltage to said control grid, a uni- 6 laterally conductive element, and means connecting said unilaterally conductive element between said cathode and said first control electrode to limit the maximum electron beam current in said cathode ray tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,226,996 Schlesinger Dec. 31, 1940 10 2,371,897 Knick Mar. 20, 1945 2,540,646 Bernard Feb. 6, 1951 2,884,562 Solow Apr. 28, 1959 

